This invention relates to a high voltage power supply which can switch a capacitive load between two voltage levels above ground voltage, and more specifically relates to a novel switching circuit which can switch energy between a power supply and a capacitive load at speeds less than 20 microseconds, at high repetition rate, and high load volt-amperes, using commercially available solid state components and in a relatively low volume with low power dissipation.
There is a growing need for high voltage power supplies which can be switched between two voltage levels above the ground potential of a load. For example, beam penetration cathode ray tubes are known which have anode voltages which are switched between two voltage levels (e.g. 12 kV and 18 kV) to excite one of two phosphor layers, or both phosphor layers which are on the face plate of the CRT. The two layers may have different characteristics, for example, different colors or different persistences so that the display information will appear in a different mode when written at the high (18 kV) or low (12 kV) anode voltage.
When the tube has a high persistence layer and low persistence layer, the tube is useful for presenting a radar display where the target position information is displayed on the long persistence phosphor (written with the 18 kV anode voltage), while identifying information written adjacent the target, such as target identification, heading, altitude, and the like is written on the short persistence phosphor (written with the 12 kV anode voltage) and is rapidly updated without interference with prior written information. In a typical radar system, target position information is written on the long persistence phosphor with a radial line scan which rotates around the display. Each radial line is written at a time depending on the range of the radar. Thus, if the radar range is 100 miles, the radial line must be written on the high persistence phosphor in about 1200 microseconds at the 18 kV anode voltage. The radial lines are then written at a repetition rate of about 3 kHz, with a recharging period of about 100 microseconds between each repetition. The writing of information on the low persistence layer takes place during this 100 microsecond recharging period at the lower 12 kV anode voltage. Thus, for the system to be operative, the power supply must be able to switch from 18 to 12 kV and back to 18 kV at the beginning and end of the 100 microsecond recharging period while still having enough time (e.g. 60 microseconds) for writing information on the low persistence display. This then leaves only 20 microseconds for switching the power supply at the beginning and end of the recharging interval. Obviously, the faster the switching time, the more information can be written.
Presently existing, solid state power supplies cannot switch at this high speed. For example, power supplies made according to the disclosure of U.S. Pat. No. 3,659,190, dated Apr. 25, 1972, in the name of Filippo B. Galluppi, and assigned to the assignee of the present invention, are commercially available as the power supply type CS 18 from Venus Scientific Inc., Farmingdale, N.Y. These power supplies can switch between 12 and 18 kV, but require about 150 microseconds to switch. If the switching time is made shorter, it would be necessary to make the components excessively large in order to handle the substantial power which would have to be dissipated in a very short time.
Consequently, the above high speed (e.g. less than about 20 microsecond switching of a capacitive load), necessary to provide annotation in a short range radar system, has required the use of special vacuum tube circuits or special information processing circuits and the like which are expensive and bulky.